Method for removing fluoride from a zinc-containing solution or suspension, defluoridated zinc sulfate solution and use thereof, and method for producing zinc and hydrogen fluoride or hydrofluoric acid

ABSTRACT

Embodiments of the invention relate to a process for removing fluoride from a solution or suspension containing zinc, in particular a solution of zinc sulfate, a defluoridated solution of zinc sulfate obtainable by such a process, its use as well as processes for producing zinc and hydrogen fluoride or hydrofluoric acid. The process for removing fluoride comprises (i) providing a solution or suspension A containing zinc, wherein the solution or suspension A containing zinc further contains fluoride ions; (ii) adding a solution B containing a dissolved salt of a rare earth element to the solution or suspension A containing zinc, wherein a solid comprising a rare earth element fluoride and a solution C containing zinc are formed; and (iii) separating the solid from the solution C containing zinc, wherein the solution C containing zinc has a lower concentration of fluoride ions than the solution or suspension A containing zinc.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is the U.S. national phase of International ApplicationNo. PCT/EP2017/058303 filed 6 Apr. 2017 which designated the U.S., theentire content of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of the invention relate to a process for removing fluoridefrom a solution or suspension containing zinc, in particular a solutionof zinc sulfate, a defluoridated solution of zinc sulfate obtainable bysuch a process as well as its use in the fertilizer industry, theviscose silk industry or feeding stuff industry or in the production offertilizers, viscose silk (rayon) or feeding stuff. Further embodimentsof the invention relate to processes for producing zinc, in particularfor the hydrometallurgical recovery of zinc, as well as to processes forproducing hydrogen fluoride or hydrofluoric acid.

BACKGROUND

For the primary zinc industry, but also for the zinc sulfate industry,such as the fertilizer industry or feeding stuff industry, secondary rawmaterials, in particular having a low iron content and a high zinccontent, become more and more important. A limiting factor in thisconnection is often the fluorine content of the secondary raw materials,for which reason the processing of secondary raw materials is generallylimited and nevertheless these secondary raw materials usually have tobe subjected to a thermal treatment, which in turn often leads todisadvantages, such as corrosive exhaust gases, as well as todifficulties in the selective recovery of lead as a byproduct. On theother hand, a too high fluorine content of the secondary raw materialsleads to massive problems in the hydrometallurgical recovery of zincwhich forms the basis of at least 85% of the global zinc production.Also in the agricultural chemistry, a drastic reduction of the fluoridecontents is of great importance for the production of mineralfertilizers with additives of zinc sulfate because cultivated plants maytolerate in general only very low amounts of soluble fluorides.

Thus, there is a need to refine fluoride-loaded zinc secondary rawmaterial, such as waelz oxides, in particular to defluoridate the same,such that they may be employed in the primary zinc industry or for theproduction of zinc sulfate without suffering from the above describedlimitations or disadvantages.

OBJECTS OF THE INVENTION

Thus, there may be a need to provide a process by means of which asolution or suspension containing zinc, which has been obtained forinstance from a fluoride-loaded zinc secondary raw material, may bedefluoridated so that such fluoride-loaded zinc secondary raw materialmay be made available for the primary zinc industry or the zinc sulfateindustry, in particular for the hydrometallurgical recovery of zinc,without the occurrence of limitations or problems due to fluoridecontents. In addition, it is aimed that such process may be carried outeconomically efficient and resource-conserving, and thus environmentallyfriendly, as possible. Moreover, there may be a need to provide asolution of zinc sulfate having a very low fluoride content so that itmay be utilized in the fertilizer industry, in precipitation baths ofthe viscose silk industry or feeding stuff industry or in the productionof fertilizers, viscose silk or feeding stuff. Furthermore, there may bea need to be able to obtain both zinc or zinc sulfate and hydrofluoricacid from a fluoride-loaded zinc secondary raw material.

SUMMARY OF THE INVENTION

The inventors of the invention have carried out extensive studies forsolving these objects and have in particular found that fluorides may beprecipitated almost quantitatively from a solution or suspensioncontaining zinc and separated in an efficient manner by means ofsolutions of salts of rare earth elements.

Accordingly, an exemplary embodiment of the invention relates to aprocess for removing fluoride from a solution or suspension containingzinc, in particular a solution of zinc sulfate, the process comprisingthe steps of:

-   -   (i) providing a solution or suspension A containing zinc,        wherein the solution or suspension A containing zinc further        contains fluoride ions;    -   (ii) adding a solution B containing a dissolved salt of a rare        earth element to the solution or suspension A containing zinc,        wherein a solid comprising a rare earth element fluoride and a        solution C containing zinc are formed;    -   (iii) separating the solid from the solution C containing zinc,        wherein the solution C containing zinc has a lower concentration        of fluoride ions than the solution or suspension A containing        zinc.

In addition, an exemplary embodiment of the invention relates to adefluoridated solution of zinc sulfate, obtainable by a process forremoving fluoride from a solution or suspension containing zincaccording to the invention, wherein the solution has a content offluoride of at most 10 mg/l.

The defluoridated solution of zinc sulfate according to the inventionmay be processed or used in various manner, for instance in thefertilizer industry, in precipitation baths of the viscose silk industryor feeding stuff industry or in the production of fertilizers, viscosesilk and feeding stuff and for the hydrometallurgical production ofzinc.

Thus, an exemplary embodiment of the invention relates to the use of adefluoridated solution of zinc sulfate according to the invention forthe production of fertilizers, viscose silk and/or feeding stuff.

Furthermore, an exemplary embodiment of the invention relates to aprocess for producing zinc from a composition containing zinc oxide, inparticular waelz oxide, the process comprising the steps of:

-   -   providing an apparatus configured for hydrometallurgical        recovery of zinc;    -   performing a hydrometallurgical recovery of zinc from a        defluoridated solution of zinc sulfate according to the        invention by means of the apparatus configured for        hydrometallurgical recovery of zinc.

A further exemplary embodiment of the invention relates to a process forproducing zinc from a composition containing zinc oxide, in particularwaelz oxide, the process comprising the steps of:

-   -   providing an apparatus configured for hydrometallurgical        recovery of zinc;    -   performing a process for removing fluoride from a solution or        suspension containing zinc according to the invention in another        apparatus;    -   performing a hydrometallurgical recovery of zinc from the        solution C containing zinc by means of the apparatus configured        for hydrometallurgical recovery of zinc.

Furthermore, an exemplary embodiment of the invention relates to aprocess for producing hydrogen fluoride or hydrofluoric acid, theprocess comprising the steps of:

-   -   performing a process for removing fluoride from a solution or        suspension containing zinc according to the invention;    -   treating the solid comprising a rare earth element fluoride with        an acid, in particular sulfuric acid, wherein hydrogen fluoride        (HF) and a solution of a salt, in particular a sulfate, of the        rare earth element are formed;    -   separating the hydrogen fluoride (HF) from the solution of the        salt of the rare earth element, for instance by means of        distillation.

Other objects and advantages of embodiments of the invention will bereadily appreciated by means of the following detailed description.

DETAILLED DESCRIPTION OF THE INVENTION

Hereinafter, details of embodiments of the invention and furtherembodiments thereof will be described. However, the invention is notlimited to the following specific descriptions, but they are rather onlyfor illustrative purposes of the inventive teachings.

It should be noted that features described in connection with oneexemplary embodiment or exemplary aspect may be combined with any otherexemplary embodiment or exemplary aspect. In particular, featuresdescribed in connection with any exemplary embodiment of the process forremoving fluoride may be combined with any further exemplary embodimentof the process for removing fluoride as well as with any exemplaryembodiment of the defluoridated solution of zinc sulfate, its use aswell as any exemplary embodiment of the processes for producing zinc andhydrogen fluoride or hydrofluoric acid, and vice versa, unlessspecifically stated otherwise.

Where an indefinite or definite article is used when referring to asingular term, such as “a”, “an” or “the”, a plural of that term is alsoincluded and vice versa, unless the context clearly dictates otherwise.The expression “comprising”, as used herein, includes not only themeaning of “comprising”, “including” or “containing”, but may alsoencompass “consisting essentially of” and “consisting of”.

In the process for removing fluoride from a solution or suspensioncontaining zinc, a solution or suspension A containing zinc is providedin a (first) process step (step (i)), wherein the solution or suspensionA containing zinc further contains fluoride ions. The solution orsuspension A containing zinc may in particular be a solution of zincsulfate.

In an exemplary embodiment, the solution or suspension A containing zincis formed by at least partly dissolving a composition containing zincoxide, in particular waelz oxide. In other words, it is possible thatthe composition containing zinc oxide is completely or partly dissolved.In case of a complete dissolution of the composition containing zincoxide, a solution A containing zinc is formed, while in case of apartial dissolution of the composition containing zinc oxide, asuspension A containing zinc is formed.

In an exemplary embodiment, the composition containing zinc oxide is afluoride-containing zinc secondary raw material, such as waelz oxide.

A “waelz oxide” within the meaning of the invention is generallyreferred to the main product of a Waelz process. The “Waelz process” isgenerally referred to a metallurgical process for recycling of wastematerials containing zinc, such as steel mill dusts, electric furnacesteel filter dusts or electric arc furnace flue dusts, in a rotary kilnor rotary furnace. Accordingly, waelz oxide within the meaning ofembodiments of the invention contains zinc oxide (ZnO), in particular asa main component thereof. In addition, waelz oxide in general furthercontains disturbing impurities, such as fluorine or fluoride, lead andothers (see also Table 1). Prior to or after the defluoridation,disturbing lead is precipitated for instance by means of H₂S or is wellcemented by means of zinc. A further development of the Waelz processwith reduced energy consumption represents the SDHL Waelz process, namedafter the inventors Saage, Dittrich, Hasche and Langbein.

In the following Table 1, typical compositions of waelz oxides from theSDHL Waelz process are summarized, wherein “waelz oxide (washed)” meansthat (unwashed) waelz oxide has been subjected to a leaching by means ofhot diluted soda solution:

TABLE 1 Typical compositions of waelz oxides from the SDHL Waelz processwt.-% waelz oxide (unwashed) waelz oxide (washed) Zn 55-65 65-68 Pb2.3-5.5 3.9-6.0 FeO 2.1-5.4 3.0-6.0 CaO 1.4-4.0 1.8-4.5 MgO 0.2-0.50.3-0.6 SiO₂ 0.2-1.5 0.4-2.0 Cl 0.1-6.4 0.05-0.2  F 0.1-0.5  0.1-0.25 S0.2-1.0 0.1-0.5 K₂O 0.05-3.9  0.04-0.1  Na₂O 0.3-3.1 0.1-0.3

Such compositions containing zinc oxide typically contain variouscomponents, as evident from the above Table 1 as an example, so that itis possible that in case of a partial dissolution of the compositioncontaining zinc oxide, the existing zinc substantially completelydissolves and the insoluble residue does not contain any or at mosttraces of zinc. However, it is also possible that in case of a partialdissolution of the composition containing zinc oxide, a part of theexisting zinc remains in the insoluble residue, in particular in theform of hardly soluble zinc salts, but wherein at least a part of theexisting zinc, in particular a predominant part of the existing zinc,for instance at least 70%, in particular at least 80%, in particular atleast 85%, in particular at least 90%, in particular at least 95% of theexisting zinc, dissolves and is consequently present as zinc ions (Zn²⁺ions) in the solution. In case of a merely partial dissolution of thecomposition containing zinc oxide, it is also possible to separate theinsoluble residue, for instance by means of filtration orcentrifugation, and to use the thus obtained (clear) solution as thesolution A containing zinc.

In a preferred embodiment, the composition containing zinc oxide issubstantially completely dissolved to give a (aqueous) solution Acontaining zinc.

In an exemplary embodiment, the composition containing zinc oxide is atleast partly dissolved by means of an acid to form the solution orsuspension A containing zinc. Sulfuric acid has turned out to representthe most versatilely suitable acid, while also other acids, inparticular other mineral acids, may be used for at least partlydissolving the composition containing zinc oxide, as long as they do notform hardly soluble salts with in particular zinc and their acidity issufficient for dissolving of in particular zinc oxide. The concentrationof the acid used is not particularly limited and is for instance in caseof sulfuric acid a 10 to 20% sulfuric acid, such as a 15% sulfuric acid.

In some embodiments, it is advantageous to carry out the at leastpartial dissolution of the composition containing zinc oxide at anelevated temperature, for instance at a temperature within the range offrom 50 to 80° C., in particular from 60 to 70° C. A (preferablycontinuous) stirring may also positively influence the at least partialdissolution of the composition containing zinc oxide. The time periodfor the at least partial dissolution of the composition containing zincoxide is not particularly limited and is for instance 10 minutes to 2hours, in particular 30 minutes to 1 hour. An end of the dissolutionprocess may be realized in that the pH value of the aqueous solutiondoes substantially not (or no longer) change with advancing duration. Byway of illustration, in an at least partial dissolution of washed waelzoxide by means of 15% sulfuric acid, a (constant) final pH value ofabout 4 is for instance reached after about 40 minutes at a temperatureof about 65° C. while permanently stirring.

In the process for removing fluoride from a solution or suspensioncontaining zinc, a solution B containing a dissolved salt of a rareearth element is added to the solution or suspension A containing zincin a further process step (step (ii)). Here, a solid comprising a rareearth element fluoride and a solution C containing zinc are formed. Thesolution C containing zinc has a lower concentration of fluoride ionsthan the solution or suspension A containing zinc.

In an exemplary embodiment, the solution B contains a dissolved sulfateof a rare earth element. It is generally possible to also use othersalts of a rare earth element (or of several rare earth elements), suchas nitrates, as long as they are highly soluble, in particular highlysoluble in water, in particular highly soluble at a neutral or weaklyacidic pH value.

It has turned out to be advantageous that the utilized rare earthelement salt completely dissolves and that thus the solution B is aclear aqueous solution of a completely dissolved salt of a rare earthelement. Hereby, it is possible to achieve an almost quantitative orsubstantially complete precipitation of the fluoride ions, which isbasically only restricted by the solubility of the formed rare earthelement fluoride, which may be calculated from the solubility product ofthe rare earth element fluoride and is slightly increased due to theneutral salt effect or the “salting in” effect. In other words, it ishereby possible to achieve the technically and physically least possibleconcentration of fluoride ions remaining in the solution C containingzinc. Moreover, it is hereby possible to adequately dose the requiredamount of rare earth element salt so as to achieve a substantialcomplete precipitation of the fluoride ions, while avoiding anunnecessary excess of the comparatively expensive rare earth elementsalt (also from the viewpoint of conserving of resources).

In some embodiments, it might be advantageous if the amount (molaramount or amount of substance) of the rare earth element salt added bymeans of the solution B is slightly hyperstoichiometric compared to thefluoride ions contained in the solution or suspension A containing zinc.For instance, the molar ratio of the ions of the rare earth element(RE^(n+)/n) (wherein RE represents a rare earth element and n representsits charge number) to fluoride ions (F), i.e.

$\frac{\left\lbrack {RE}^{n +} \right\rbrack}{n \times \left\lbrack F^{-} \right\rbrack},$

is in the range of from 1.01 to 1.5, in particular of from 1.02 to 1.4,in particular of from 1.03 to 1.3, such as from 1.05 to 1.2. An excessof the rare earth element salt results in a further shift of theprecipitation equilibrium towards the precipitate of the rare earthelement fluoride and thus to a particular effective precipitation of thefluoride ions and corresponding removal of the fluoride ions from thesolution or suspension A containing zinc. If a remaining (i.e. notconsumed by the fluoride ions) excess of the ions of the rare earthelement is disturbing in the further use of the solution or suspension Acontaining zinc, it is advantageous to precipitate these excessive ionsof the rare earth element by means of a salt whose anion forms a lesshardly soluble rare earth element salt than rare earth element fluoride(so as to avoid the release of fluoride ions from the rare earth elementfluoride again). As an example for such a salt, mention can be made ofin particular ammonium oxalate whose anion (i.e. oxalate) forms a rareearth element oxalate being less hardly soluble rare earth element saltthan the respective rare earth element fluoride.

In an exemplary embodiment, an amount of substance of the dissolved saltof a rare earth element is therefore added in step (ii), the amountbeing adapted to an estimated (for instance based on empirical valuesfrom previously used comparable zinc secondary raw materials) orpreviously determined (for instance determined by means of ananalytically determined fluoride content in a previously retrievedsample of the solution or suspension A containing zinc, e.g.potentiometrically determined by means of a fluoride sensitiveelectrode) amount of substance of fluoride ions in the solution orsuspension A containing zinc. In this regard, it is advantageous toadapt such that the added amount (molar amount or amount of substance)of the rare earth element salt is slightly hyperstoichiometric comparedto the estimated or previously determined amount of substance offluoride ions in the solution or suspension A containing zinc, asexplained in detail above. As also explained above, such adaptation oradequate metering is facilitated, if not even only made passible, if thesolution B is a clear aqueous solution of a completely dissolved salt ofa rare earth element.

In an exemplary embodiment, the rare earth element is selected from thegroup consisting of scandium (Sc), yttrium (Y), lanthanum (La), cerium(Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). Withregard to the availability, the rare earth element is preferablyselected from the group consisting of lanthanum (La), cerium (Ce),praseodymium (Pr) and neodymium (Nd). In particular, it has turned outto be advantageous for cost efficiency if the rare earth elementcomprises cerium (Ce).

Combinations of rare earth element salts may also be used, for instancecombinations of rare earth element salts having different rare earthelements but the same anion, combinations of rare earth element saltshaving the same rare earth element but different anions, andcombinations of rare earth element salts having different rare earthelements and different anions.

In an exemplary embodiment, the salt of the rare earth element is ceriumsulfate, such as in particular cerium(III) sulfate (Ce₂(SO₄)₃). In otherwords, the solution B contains for instance dissolved cerium sulfate.

For example, in case that the solution B contains cerium(III) sulfate(Ce₂(SO₄)₃), the precipitation reaction may be illustrated by thefollowing reaction equation (1):

Ce₂(SO₄)₃+6F⁻→2CeF₃+3SO₄ ²⁻  (1)

In an exemplary embodiment, after adding the solution B to the solutionor suspension A containing zinc, the pH value of a thus obtained mixtureis buffered within the range of from 5.0 to 5.5. In such a weakly acidicto neutral environment, the precipitation of the fluoride ions by meansof the rare earth element salt may be carried out in a particularefficient manner and it is in particular possible to achieve an almostquantitative or substantially complete precipitation of the fluorideions in this environment.

Buffering of the pH value to a range of from 5.0 to 5.5 may be made forinstance by means of the addition of zinc oxide (ZnO) as buffersubstance (buffering agent), in particular in case the solution orsuspension A containing zinc has been prepared from a compositioncontaining zinc oxide by at least partial dissolution by means of anacid and thus rather has a more acidic pH value than 5.0 to 5.5. Here,the zinc oxide is added for instance to the solution or suspension Acontaining zinc, to the solution B and/or to the obtained mixture. Theaddition of zinc oxide may be made as long as the pH value is adjustedwithin the desired range. The zinc oxide may be added for instance inthe form of a freshly prepared, fluoride-free zinc oxide suspension(“zinc oxide milk”). In some embodiments, it is advantageous to applythe zinc oxide in a slight excess. Hereby, it is generally possible toimprove the filterability of the rare earth element fluoride which isalways very fine grained and thus difficult to filtrate.

Alternatively, buffering of the pH value is also possible by means ofother buffer substance, such as basic zinc carbonate, as long as thebuffer substances do not negatively influence the precipitation of therare earth element fluoride or do not disturb in the further processingor further use of the solution A containing zinc and/or of the solid.

In an exemplary embodiment, the addition of the solution B and thus alsothe incipient precipitation of the rare earth element fluoride iscarried out at an elevated temperature, in particular within the rangeof from 70 to 805° C., in particular within the range of from 75 to 84°C., in particular within the range of from 77 to 83° C., in particularat about 80° C. This is advantageous in terms of a good separability, inparticular filterability, of the formed solid.

In some embodiments, it is advantageous if the addition of the solutionB to the solution or suspension A containing zinc is carried out slowly,for example in small portions while permanently stirring. In particular,it is hereby possible to repress the nucleation rate and to promote thecrystal growth rate.

In some embodiments, it is advantageous to subject the solid comprisinga rare earth element fluoride, such as the precipitated cerium fluoride(CeF₃), to a maturation process or aging process, for instance at atemperature within the range of from 70 to 85° C., in particular of from75 to 80° C., while permanently stirring, for instance for a time periodof from 30 minutes to 4 hours, in particular of from 1 to 3 hours, andonly after a further time period of for instance from 12 to 18 hours, inparticular of from 14 to 16 hours, while allowing the solid and thesolution C containing zinc to cool down, to carry out a separation ofthe solid from the solution C containing zinc according to step (iii).Due to the maturation process or aging process, the crystals of thesolids coarsen. Hereby, the separation of the solid from the solution Ccontaining zinc, in particular by means of filtration through a (finepored) filter is facilitated.

In an exemplary embodiment, small amounts (for instance traces) of Fe³⁺ions are present in or are added to, in particular in the form of ferricsulfate (Fe₂(SO₄)₃), the solution or suspension A containing zinc and/orthe solution B. Hereby, it is possible to facilitate the sedimentationbehavior of the (cryptocrystallinely to amorphously) precipitating rareearth element fluoride, such as cerium fluoride, in particular toimprove its filterability. Without wishing to be bound to a theory, theinventors assume that the ferric sulfate hydrolyses in a weakly acidicprecipitation mixture and, in doing so, forms basic ferric sulfatesenveloping the rare earth element fluoride particles, in particular theCeF₃ particles, and thus promote their sedimentation. In an exemplaryembodiment, Fe³⁺ ions, in particular basic ferric sulfate (Fe₂O(SO₄)₂),are also formed from ferrous (Fe(II)) contained in traces in thesolution or suspension A containing zinc and/or the solution B byoxidation, for instance by means of hydrogen peroxide (H₂O₂), e.g. byaddition of some drops of perhydrol (33% H₂O₂), as illustrated by thefollowing reaction equation (2):

2Fe(SO₄)+H₂O₂Fe₂O(SO₄)₂+H₂O  (2)

The solution C containing zinc has a lower concentration of fluorideions than the solution or suspension A containing zinc.

In an exemplary embodiment, the solution or suspension A containing zinchas a content of fluoride of at least 17 mg/l, in particular of at least25 mg/l, in particular of at least 50 mg/l. When using such afluoride-loaded solution or suspension containing zinc as an electrolytein the hydrometallurgical recovery of zinc, massive problems may occur.In particular, there is a risk that an undisturbed stripping of the zinccoating deposited in the course of the electrolysis on a cathode is notpossible or that the stripping is extremely hindered.

In an exemplary embodiment, the solution C containing zinc has a contentof fluoride of at most 10 mg/l, in particular of at most 8 mg/l, inparticular of at most 6 mg/l. Hereby, it is to possible to ensure thatthe problems and disadvantages described above in connection with theuse of a fluoride-loaded solution or suspension containing zinc aresubstantially avoided.

In the process for removing fluoride from a solution or suspensioncontaining zinc, the formed solid comprising a rare earth elementfluoride is separated from the solution C containing zinc by means offor instance filtration or centrifugation in a further process step(step (iii)).

In an exemplary embodiment, the separation of the solid from thesolution C containing zinc is carried out such that subsequently boththe solution C and the solid comprising a rare earth element fluoridemay be further processed. This is advantageous in particular for costefficiency, but also for conserving resources and thus for environmentalreasons.

In an exemplary embodiment, the separation of the solid from thesolution C containing zinc comprises at least a filtration process.Hereby, the solid may be separated from the solution C containing zincin a particular simple and efficient manner and, in doing so, both thesolution C containing zinc and the solid comprising a rare earth elementfluoride are available for further processing or further use. Thefiltration process is not particularly limited. In particular, the poresize of the filter used may be selected depending on the particle sizeof the solid so as to achieve a substantially complete retention of thesolid with an acceptable flow rate. Alternatively, the separation of thesolid from the solution C containing zinc may also be carried out bymeans of centrifugation, allowing to settle and/or decanting.

The solution C containing zinc obtained after step (iii), issubstantially freed from solids or turbidities and in particularrepresents a substantially clear solution having a—apart from dilutioneffects—substantially unchanged content of zinc compared to the solutionor suspension A containing zinc, but having a reduced content offluorides. The solution C containing zinc may thus be referred to adefluoridated solution C containing zinc, in particular a defluoridatedsolution of zinc sulfate.

Before discussing a potential further processing or further use of thethus obtained solution C containing zinc (see further below), thepotential further processing or further use of the separated solidcomprising a rare earth element fluoride is described in the following.

In an exemplary embodiment, a salt of a rare earth element is formedfrom the separated solid, which salt may be reused for the solution B.Thus, the salt of a rare earth element used in step B may be reusedagain and again and it is thus possible that the used salt of a rareearth element is not consumed, but is subject to a circuit (recycled).This is enormously advantageous in particular for cost efficiency, butalso for conserving resources and thus for environmental reasons.

In an exemplary embodiment, the separated solid comprising a rare earthelement fluoride is in particular reacted with an acid, such as sulfuricacid, which may also be referred to as a “displacement acid”, so that asalt of a rare earth element, such as a sulfate of a rare earth element,is formed from the rare earth element fluoride, which may be reused forthe solution B. By doing so, zinc oxide that might also be contained inthe separated solid also dissolves, for instance in the form of zincsulfate, and may be as such also added to the solution B. The same holdstrue for possible iron salts that might be contained in the separatedsolid, which dissolve for instance in the form of ferreous sulfate andwhich may be as such also added to the solution B. In an exemplaryembodiment, the separated solid is reacted with semi-concentratedsulfuric acid (e.g. having a content of about 40 to 60 wt.-% H₂SO₄)while heating.

The following reaction equation (3) illustrates the exemplary reactionof cerium fluoride (CeF₃) with sulfuric acid:

2 CeF₃+3 H₂504 Ce₂(SO₄)₃+6 HF  (3)

As it is evident from the reaction equation (3), in addition to the saltof a rare earth element (in the present case, cerium(III) sulfate) whichmay be reused for the solution B, hydrogen fluoride (HF) is also formedwhich may be recovered.

Therefore, an exemplary embodiment of the invention also relates to aprocess for producing hydrogen fluoride or hydrofluoric acid, theprocess comprising the steps of:

-   -   performing a process for removing fluoride from a solution or        suspension containing zinc according to the invention;    -   treating the solid comprising a rare earth element fluoride with        an acid, in particular sulfuric acid, wherein hydrogen fluoride        (HF) and a solution of a salt, in particular a sulfate, of the        rare earth element are formed;    -   separating the hydrogen fluoride (HF) from the solution of the        salt of the rare earth element.

In an exemplary embodiment, the separation of the hydrogen fluoride (HF)from the solution of the salt of the rare earth element comprises adistillation process. Here, the hydrogen fluoride is distilled off ashydrogen fluoride vapour and collected in a receiver containing water togive hydrofluoric acid. Because of the very aggressive and stronglyetching quality of hydrogen fluoride or hydrofluoric acid, it isadvantageous if the distillation apparatus is made of a materialresistant to hydrogen fluoride or hydrofluoric acid, such as of Teflon(polytetrafluoroethylene).

Hydrogen fluoride or hydrofluoric acid is a versatilely usable chemicalwhich may be sold profitably.

In an exemplary embodiment, the separated solution of the salt of therare earth element is reused as a component of the solution B so that asubstantially closed circuit (loop) with regard to salt of the rareearth element may be realized.

As already announced above, the defluoridated solution C containingzinc, in particular a defluoridated solution of zinc sulfate, may befurther processed or used.

Thus, an exemplary embodiment of the invention also relates to adefluoridated solution of zinc sulfate, obtainable by a process forremoving fluoride from a solution or suspension containing zincaccording to the invention, wherein the solution has a content offluoride of at most 10 mg/l.

In an exemplary embodiment, the defluoridated solution of zinc sulfatehas a content of fluoride of at most 9 mg/l, in particular of at most 8mg/l, in particular of at most 7.5 mg/l, in particular of at most 7mg/l, in particular of at most 6 mg/l.

The lower limit of the content of fluoride in particular depends on thesolubility of the rare earth element fluoride formed in the course ofthe process, as described in detail above in connection with the almostquantitative or substantially complete precipitation of the fluorideions, wherein of course contents of fluoride below the solubilitycalculated from the solubility product may be achieved by diluting thesolution of zinc sulfate with water. For example, the solubility ofcerium fluoride (CeF₃) is 4.22 mg/l. In an exemplary embodiment, thedefluoridated solution of zinc sulfate then has a content of fluoride ofat least 1 mg/l, in particular of at least 2 mg/l, in particular of atleast 3 mg/l, in particular of at least 4 mg/l, in particular of atleast 4.5 mg/l.

The defluoridated solution of zinc sulfate may be processed or used invarious manner, for instance in the fertilizer industry, inprecipitation baths of the viscose silk industry or feeding stuffindustry or in the production of fertilizers, viscose silk and feedingstuff and for the (in particular hydrometallurgical) production of zinc.

Thus, an exemplary embodiment of the invention also relates to the useof a defluoridated solution of zinc sulfate according to the inventionfor the production of fertilizers, viscose silk and/or feeding stuff.

A further exemplary embodiment of the invention further relates to aprocess for producing zinc from a composition containing zinc oxide, inparticular waelz oxide, the process comprising the steps of:

-   -   providing an apparatus configured for hydrometallurgical        recovery of zinc;    -   performing a hydrometallurgical recovery of zinc from a        defluoridated solution of zinc sulfate according to the        invention by means of the apparatus configured for        hydrometallurgical recovery of zinc.

A still further exemplary embodiment of the invention relates to aprocess for producing zinc from a composition containing zinc oxide, inparticular waelz oxide, the process comprising the steps of:

-   -   providing an apparatus configured for hydrometallurgical        recovery of zinc;    -   performing a process for removing fluoride from a solution or        suspension containing zinc according to the invention in another        apparatus;    -   performing a hydrometallurgical recovery of zinc from the        solution C containing zinc by means of the apparatus configured        for hydrometallurgical recovery of zinc.

In order not to hinder the hydrometallurgical zinc recovery process, theremoval of fluoride is carried out in a separate apparatus, for examplein a separate heatable agitator vessel, in parallel connection.

Embodiments of the invention are further described by means of thefollowing examples, which are solely for the purpose of illustrating theinventive teachings, and shall not be construed as limiting the scope ofthe invention in any way.

EXAMPLES

A stock solution having a zinc content of 45 g/I was prepared fromwashed waelz oxide having a composition as shown in the following Table2 by neutral leaching using diluted (15%) sulfuric acid.

TABLE 2 Composition of the waelz oxide used for the defluoridationexperiments Component Content [wt.-%] Component Content [wt.-%] F 0.24Mg 0.18 Cl 0.1 Mn 0.22 SO₄ 0.44 Pb 4.8 Ca 1.1 Zn 67.0 Fe 2.8 Si 0.22

The concentration of zinc was adjusted by reacting 79.017 g waelz oxidewith 450.06 g H₂SO₄ (15%), that is 409.1 ml.

By doing so, 85% of the zinc oxide of the furnished amount of waelzoxide dissolved. This leaching corresponds to a neutral leaching with afinal pH value of 4. With these leaching conditions, a partialdissolution of silicates (from gangue parts upon forming of colloidalsilicic acid clogging filter pores and thus hindering the filtrationprocess after the neutral leaching) was avoided. During the neutralleaching, the suspension was continuously stirred. The leachingtemperature was 65° C., the leaching time was 40 minutes. The end of theneutral leaching could be observed by pH constancy.

After completion of the neutral leaching, the residue of the neutralleaching was separated from the mother liquor by filtration. Thecolorless and clear filtrate was transferred in a 1 liter volumetricflask and diluted until the measuring mark. The content of the 1 litervolumetric flask was transferred in two 500 ml volumetric flasks.

The content of one 500 ml volumetric flask was used for thedetermination of the zinc and fluorine contents of the stock solution,the content of the second 500 ml volumetric flask was transferred in a800 ml beaker and slowly, dropwise, mixed with a slightlyhyperstoichiometric amount of cerium(III) nitrate dissolved in about 50ml water. The stoichiometricially required amount of Ce(NO₃)₃.6H₂O wascalculated from the fluorine content of the waelz oxide as follows:

(0.24/100)×79.017×0.5=0.09482 g or 94.82 mg fluorine

This amount of fluorine requires:

94.82×(434.22/56.995)=722.4 mg Ce(NO₃)₃.6H₂O

Explanation of the numbers:

-   0.24=fluorine content of the waelz oxide [wt.-%]-   434.22=molar mass of Ce(NO₃)₃.6H₂O [g/mol]-   79.017=weighted sample of waelz oxide [g]-   56.995=3 times the molar mass of fluorine [g/mol]

In the course of the dropwise addition of the Ce(NO₃)₃ solution, thesolution became turbid by precipitated cerium(III) fluoride. In order toimprove its sedimentation, a few drops of perhydrol (H₂O₂, 33%) wasadded to the suspension. In doing so, ferrous (Fe(II)) present in tracesin the suspension oxidized to give basic ferric sulfate according to thereaction equation (2).

Since basic ferric sulfate tends to hydrolyze and in doing so formssulfuric acid, some basic zinc carbonate was added to the suspension. Asa consequence, the slightly fallen pH value was raised again to thevalue of 5.

The suspension thus obtained was stirred for 2 hours at a temperature offrom 70 to 80° C. and only thereafter cooled down. The suspension wasallowed to stand for 14 hours for sedimenting the solid. Onlythereafter, the solid brownish colored by FeO(OH) was filtered off andwashed with cold water.

The filtrate was transferred again in a 500 ml volumetric flask anddiluted until the measuring mark. Subsequently, the zinc and fluorinecontents of the thus obtained solution were determined. In line with theabove described preparation and measurement scheme, all in all 3defluoridation experiments were carried out. The results of themeasurements can be taken from the following Table 3.

TABLE 3 Results of the defluoridation experiments by precipitatingfluoride ions from a zinc sulfate solution by means of Ce³⁺ ions Contentof the zinc sulfate solution Before the precipitation After theprecipitation Experiment 1 Zn 48.1 g/l 45.8 g/l F 54.0 mg/l  6.0 mg/lExperiment 2 Zn 48.4 g/l 48.4 g/l F 59.0 mg/l  4.0 mg/l Experiment 3 Zn  50 g/l 49.8 g/l F   63 mg/l  4.5 mg/l

As it is evident from the measurement results, the zinc contents of thesample solutions are not significantly influenced by the precipitationof fluoride. The zinc contents of the sample solutions before and afterthe precipitation of fluoride are substantially the same.

The fluoride contents of the sample solutions to be defluoridated arehowever largely lowered by the addition of a stoichiometric (or aslightly hyperstoichiometric) amount of Ce³⁺ ions in a weakly acidic toneutral solution.

Even though the equilibrium solubility calculated from the solubilityproduct is not always achieved because of the neutral salt effect,nevertheless final fluoride contents are achieved that allow for anundisturbed stripping of the zinc coating from the Al cathodes in thehydrometallurgical recovery of zinc.

It is thus possible to refine and use fluoride-containing zinc secondaryraw materials, such as washed waelz oxide, for the hydrometallurgicalrecovery of zinc by means of the process for removing fluoride from asolution or suspension containing zinc according to the invention.

Embodiments of the invention have been described by way of specificembodiments and examples. The invention is however not limited theretoand various modifications thereof are possible, without departing fromthe scope of the invention.

1.-15. (canceled)
 16. A process for removing fluoride from a solution orsuspension containing zinc, the process comprising the steps of: (i)providing a solution or suspension A containing zinc, wherein thesolution or suspension A containing zinc further contains fluoride ions;(ii) adding a solution B containing a dissolved salt of a rare earthelement to the solution or suspension A containing zinc, wherein a solidcomprising a rare earth element fluoride and a solution C containingzinc are formed; (iii) separating the solid from the solution Ccontaining zinc, wherein the solution C containing zinc has a lowerconcentration of fluoride ions than the solution or suspension Acontaining zinc.
 17. The process according to claim 16, wherein thesolution or suspension A containing zinc is formed by at least partlydissolving a composition containing zinc oxide, in particular waelzoxide, wherein the composition containing zinc oxide is at least partlydissolved to form the solution or suspension A containing zinc.
 18. Theprocess according to claim 16, wherein the solution B contains adissolved sulfate of a rare earth element.
 19. The process according toclaim 16, wherein the rare earth element is selected from the groupconsisting of lanthanum (La), cerium (Ce), praseodymium (Pr) andneodymium (Nd).
 20. The process according to claim 16, wherein in step(ii) an amount of substance of the dissolved salt of a rare earthelement is added, the amount being adapted to an estimated or previouslydetermined amount of substance of fluoride ions in the solution orsuspension A containing zinc.
 21. The process according to claim 16,wherein, after adding the solution B to the solution or suspension Acontaining zinc, the pH value of a thus obtained mixture is bufferedwithin a range of from 5.0 to 5.5.
 22. The process according to claim16, wherein the separation of the solid from the solution C containingzinc is carried out such that subsequently both the solution C and thesolid may be further processed.
 23. The process according to claim 16,wherein the solution or suspension A containing zinc has a content offluoride of at least 17 mg/l.
 24. The process according to claim 16,wherein a salt of a rare earth element is formed from the separatedsolid, which salt may be reused for the solution B.
 25. A defluoridatedsolution of zinc sulfate, obtainable by a process according to claim 16,wherein the solution has a content of fluoride of at most 10 mg/l. 26.The defluoridated solution of zinc sulfate according to claim 25 for usein the production of fertilizers, viscose silk and/or feeding stuff. 27.The process according to claim 17, wherein the composition containingzinc oxide is at least partly dissolved by means of an acid, inparticular sulfuric acid, to form the solution or suspension Acontaining zinc.
 28. A process for producing zinc from a compositioncontaining zinc oxide, the process comprising the steps of: providing anapparatus configured for hydrometallurgical recovery of zinc; performinga process for removing fluoride from a solution or suspension containingzinc according to claim 16 in another apparatus; performing ahydrometallurgical recovery of zinc from the solution C containing zincby means of the apparatus configured for hydrometallurgical recovery ofzinc.
 29. The process according to claim 16, wherein the dissolved saltof a rare earth element is cerium sulfate.
 30. The process according toclaim 16, wherein the rare earth element comprises cerium (Ce).
 31. Theprocess according to claim 21, wherein the pH value is buffered withinthe range of from 5.0 to 5.5 by means of addition of zinc oxide (ZnO).32. The process according to claim 16, wherein the separation of thesolid from the solution C containing zinc comprises at least afiltration process.
 33. The process according to claim 16, wherein thesolution C containing zinc has a content of fluoride of at most 10 mg/l.34. The process according to claim 28, wherein the compositioncontaining zinc oxide comprises waelz oxide and wherein the step ofperforming a process for removing fluoride from a solution or suspensioncontaining zinc in another apparatus comprises producing a solution ofzinc sulfate from waelz oxide and circulation sulfuric acid
 35. Theprocess according to claim 16, wherein the process for removing fluoridefrom a solution or suspension containing zinc is a process for removingfluoride from a solution of zinc sulfate.